Toxic dinoflagellates of the genus Alexandrium occurring along the northeastern coasts of the United States and Canada pose an important problem in population biology and systematics, as well as a serious economic and public health concern. The organisms responsible for causing the toxic outbreaks have apparently spread from endemic populations in eastern Canada as far south as Long Island over the last few decades (Hayhome, B. A., et al., Mar. Biol. 101: 427-435 (1989)). Identification of different populations of these toxic algae is essential in understanding and managing the regional phenomenon.
Toxic red tides along the New England and Canadian coasts result from "blooms" of Alexandrium fundyense and A. tamarense. These dinoflagellates produce potent neurotoxins which can accumulate in shellfish. Ingestion of contaminated shellfish by human consumers can cause paralysis and in extreme cases death, giving rise to the term "paralytic shellfish poisoning", or PSP. PSP toxins were documented in northern Maine in 1958, New Hampshire and Massachusetts in 1972, Connecticut in 1982, and Long Island in 1986. This has been interpreted as evidence of an introduction, dispersal, and successful colonization of these toxic species into New England's coastal waters from source populations long-established in eastern Canada. Similar expansions in the geographic range of toxic Alexandrium species have occurred in the Pacific northwest, and in other parts of the world. The success of this organism's dispersal and recurrence appear to lie in its ability to undergo a sexual phase of its life cycle and over winter in sediments as a zygotic dormant cell or cyst. In spring, these cysts can germinate, presumably undergo meiotic divisions and resume asexual growth, populating the overlying waters (Anderson and Morel, Marine Sci. 8: 279-293 1979; Anderson et al., Mar. Biol. 76: 179-189, 1983; Fritz, et al., J. Phycology 25: 95-107 1989).
It is now evident that Alexandrium populations throughout the world are not homogeneous in their distribution, but rather consist of sub-populations with different physiological characteristics (Maranda, L., et al., Estur. Coastal Shelf Sci. 21: 401-410 (1985); Hayhome, B. A., et al., Mar. Biol. 101: 427-435 (1989)) and possible differences with respect to their origin or timing of their dispersal (Anderson, D. M., In Okaichi, T., Anderson, D. M. and Nemoto, T. (Eds.) Red tides: Biology, environmental science and toxicology. Elsevier New York, pp. 11-20 (1989). It is also of note that Alexandrium blooms and cyst accumulations can be widespread in some coastal waters, but patchy and localized within shallow embayments and estuaries in other areas (Anderson et al., Estur. Coastal Shelf Sci. 14: 447-458 (1982), Schrey et al., 1984, Estuaries 7: 472-477 (1984)). Blooms can thus be considered as "point sources", with little or no exchange or mixing with other Alexandrium populations, or on the other hand, can cover hundreds of kilometers and can be advected far from their origins, providing greater opportunity for gene flow among populations. Toxicity records compiled by the state governments support the concepts of both localized, independently blooming populations and advection of larger blooms. Despite the excellent progress made in understanding Alexandrium bloom dynamics, the source of the cells responsible for a given PSP outbreak and the degree of their genetic homogeneity remain enigmas.
The taxonomy of the armoured, saxitoxin-producing dinoflagellates (historically called the "tamarensis/catenella group") has long been confusing and contentious. For years controversy centered around an appropriate genus designation, and only recently was an international agreement made to use Alexandrium (Steidinger and Moestrup, In: Toxic Marine Phytophankton, Ed. by E. Granch; et al., pp. 522-523 (1990)). Although the confusion over genus names appears to be over, there continue to be concerns about species assignments. On one side are those that differentiate species on the basis of small morphological features such as the shapes and positions of certain thecal plates or the presence or absence of pores (Balech, In: Toxic Dinoflagellates. Anderson, D. M., et al., (Eds.) Elsevier, New York, pp. 33-38 (1985)). At the other side of the controversy are those who believe the tamarensis/catenalla group represents a single species complex comprised of numerous biochemically-distinct varieties (Taylor, F. J. R. "The taxonomy and relationships of red tide dinoflagellates, In: Toxic Dinoflagellates., Anderson, D. M., et al. (Eds.) Elsevier, New York, pp. 11-26 (1985)); Cembella, A. D. et al., Biochem. Sys. and Ecol. 14: 311-323 (1986); and Cembella, A. D., et al., Biochem. Sys. and Ecol. 15: 171-186 (1987). The generation of additional, independent criteria for examining different isolates could aid in the settlement of this controversy, and benefit those interested in unifying Alexandrium systematics.
Detailed toxin composition and enzyme electrophoretic studies, in conjunction with traditional, morphologically-based taxonomic analyses, have all been applied to assess the genetic similarity of Alexandrium isolates. Collectively, these markers represent complex character states which are dependent on the coordinated expression of multiple genes and, consequently, require fastidious culturing, harvesting, preparation and analytical procedures to ensure equitable comparisons. Despite such efforts, the delineation of population and taxonomic boundaries within and between Alexandrium species has remained coarse. At present, there is both a lack of specific genetic markers for many Alexandrium strains, as well as international disagreement over the relative importance of morphologically-based taxonomic criteria. This, in turn, has complicated efforts to understand the population dynamics and potential dispersal of these toxic organisms.